1. Field of the Invention
The present invention relates to a liquid discharge apparatus for forming an image on a recording medium by discharging a liquid. In particular, the present invention relates to a liquid discharge apparatus of the type in which the maintenance is performed in accordance with the purge operation. In the purge operation, the liquid in a sub tank is pressurized to be fed to a discharge head, and the liquid in the discharge head is forcibly discharged from the discharge head.
2. Description of the Related Art
A recording apparatus based on the ink-jet system is known as an example of the liquid discharge apparatus. The recording apparatus forms an image on a recording medium such as the recording paper by discharging the ink from the recording head. The discharge liquid such as the ink is viscous. Therefore, it is necessary to perform the maintenance for the recording head every time when a predetermined period of time has elapsed or when the instruction is made by a user. Examples of the maintenance are disclosed in Japanese Patent Application Laid-open No. 2001-212982 and U.S. Pat. No. 6,837,560 (corresponding to Japanese Patent Application Laid-open No. 2004-58663).
That is, Japanese Patent Application Laid-open No. 2001-212982 discloses a recording apparatus based on the so-called tube system in which an ink tank and a recording head are communicated with each other by a tube, and the ink is supplied from the ink tank to the recording head via the tube. In the case of this recording apparatus, a sensor is provided to detect the residual amount or residual quantity of the ink contained in the ink tank. The purge operation is executed by supplying the ink at a pressure corresponding to the residual amount of the ink detected by the sensor.
On the other hand, U.S. Pat. No. 6,837,560 discloses a recording apparatus based on the so-called station system (also referred to as “on-demand system”) in which the ink is supplied from a main tank to a sub tank connected to a recording head. In the case of this recording apparatus, on supplementing the sub tank with the ink from the main tank when an image recording is performed, a maintenance for the interior of the recording head is performed by covering a discharge port of the recording head with a cap and sucking or pressurizing the ink in the recording head.
In the case of the maintenance for the liquid discharge apparatus based on the station system, unlike the example described above, the purge operation is sometimes performed such that the interior of the sub tank is pressurized to forcibly supply the liquid toward the discharge head thereby the liquid is discharged from the discharge holes of the discharge head. If the maintenance is performed in accordance with the purge operation as described above, when the pressurization is performed, the internal pressure of the sub tank is changed depending on the residual amount of the ink contained in the sub tank.
For example, when the residual amount of the ink is small (when the volume of the sub tank not occupied by the ink is large), there is a possibility that the internal pressure of the sub tank required for the purge operation cannot be raised sufficiently, and it is impossible to perform any appropriate maintenance. On the other hand, if a large-sized pump is installed in order to raise the internal pressure, then the liquid discharge apparatus is large-sized, and the cost becomes expensive, which is not preferred.